Correlation of ammonia and blood laboratory parameters with hepatic encephalopathy: A systematic review and meta-analysis

Background and objectives Emerging research suggests that hyperammonemia may enhance the probability of hepatic encephalopathy (HE), a condition associated with elevated levels of circulating ammonia in patients with cirrhosis. However, some studies indicate that blood ammonia levels may not consistently correlate with the severity of HE, highlighting the complex pathophysiology of this condition. Methods A systematic review and meta-analysis through PubMed, Scopus, Embase, Web of Science, and Virtual Health Library were conducted to address this complexity, analyzing and comparing published data on various laboratory parameters, including circulating ammonia, blood creatinine, albumin, sodium, and inflammation markers in cirrhotic patients, both with and without HE. Results This comprehensive review, which included 81 studies from five reputable databases until June 2024, revealed a significant increase in circulating ammonia levels in cirrhotic patients with HE, particularly those with overt HE. Notably, significant alterations were observed in the circulating creatinine, albumin, sodium, interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNFα) in HE patients. Conclusions These findings suggest an association between ammonia and HE and underscore the importance of considering other blood parameters such as creatinine, albumin, sodium, and pro-inflammatory cytokines when devising new treatment strategies for HE.

sodium levels were significantly lower.Therefore, the authors concluded that relying solely on targeting ammonia in the treatment of hepatic encephalopathy might not be sufficient and suggested a new strategy involving addressing inflammation, creatinine, albumin, and sodium concentrations.This conclusion is supported by a comprehensive examination of numerous cases using appropriate methods.
However, there are several points that warrant further discussion.The authors also note significant variability in ammonia levels.One possible cause is the variation in ammonia reference values among studies.In multicenter studies involving ammonia levels, some use ratios to the upper limit of the reference value rather than raw data.It would be beneficial to address how this aspect is evaluated.Blood ammonia levels are not necessarily an appropriate indicator for evaluating hepatic encephalopathy, partly because many of the examined ammonia values are from venous blood samples.
Venous blood ammonia, being detoxified through the glutamine synthesis pathway in skeletal muscles, tends to be lower than arterial blood levels.However, since the ammonia entering the brain is transported by arterial blood, venous blood ammonia levels may not accurately reflect the concentration of ammonia flowing into the brain.This issue also warrants discussion in the analysis.
Our response: Thank you for your valuable comment.Based on various clinical studies, it has been observed that both venous and arterial ammonia levels have strong correlations with the severity of hepatic encephalopathy.Some studies even suggest a venous sample is adequate for measuring ammonia levels.Additionally, since arterial ammonia levels are higher than venous levels, it is believed that the venous level can accurately represent the level that may lead to brain injury, as arterial ammonia reaches the brain more quickly.Therefore, venous ammonia levels can be considered as reliable biomarkers.Thus, it appears that our systematic review and meta-analysis can overlook the kind of arterial and venous sampling used for ammonia measurement.Mehmood, M. A., T. Waseem, F. Z. Ahmad, and M. A. Humayun."Measuring partial pressure of ammonia in arterial or venous blood vs total ammonia levels in hepatic encephalopathy."J.

Reviewer #2:
In this paper, Sepehrinezhad A and coll.attempted to synthetize the correlation of blood ammonia with hepatic encephalopathy in a meta-analysis.I have major concerns about the methodology of this paper, particularly regarding the literature search, the criteria for inclusion and exclusion of studies which are missing and the statistical analyses.Key search terms must be noted in the main text and must be combined within each database (not done), abstracts from liver congresses are usually screened (not done), literature search must be updated in April 2024 and some studies were not identified.In order to reduce risk of bias, strict criteria for inclusion and exclusion of studies must be clearly defined prior to the literature search.Also, aims of the study and endpoints are missing in the text.The selection of the studies for inclusion in the metaanalysis includes usually 4 processes which is mandatory to preserve: identification, screening, eligibility (missing) and inclusion (Fig 1).Regarding statistical analysis, I2 alone is not enough to assess heterogeneity between studies.Moreover, in cases of moderate or high heterogeneity, the methodological section of each study is usually re-reviewed to determine whether any discrepancy could be identified, and sensitivity analyses excluding the discrepant study is classically performed (not done).Therefore, because the methodology and statistical analyses were not complete and rigorous, I recommend rejecting this article.
Our response: I appreciate your attention to detail and thoughtful critique of the methodology used in this study.As you we have suggested, expanded our literature search to include publications up to June 2024 and have incorporated our key search terms into the main body of the revised manuscript.The primary keyword search pattern for each database is now available in Supplementary Table 2.For the following comment "abstracts from liver congresses are usually screened (not done)", as we mentioned in the Prisma flowchart, we excluded abstracts from the conference because they didn't have enough data for concluding as well as there weren't inclusion criteria for the conference paper.
For the comment "In order to reduce risk of bias, strict criteria for inclusion and exclusion of studies must be clearly defined prior to the literature search", I would like to confirm that we have implemented these measures.We established general exclusion criteria, which included non-English papers, abstracts, and reviews.Additionally, we only included studies that met the following specific criteria: i) studies that compared cirrhosis with hepatic encephalopathy, ii) the use of a valid evaluation method, iii) a valid and clear definition of cases, and iv) the measurement of outcomes using a reproducible and reliable method.
To further minimize the risk of bias, we conducted an appraisal utilizing the Joanna Briggs Institute (JBI) critical appraisal checklist for case-control studies.This approach shows that an appraisal checklist can effectively mitigate the risk of bias in studies with poor methodology.
In the comment, "The selection of the studies for inclusion in the meta-analysis involves four mandatory processes: identification, screening, eligibility (missing), and inclusion (Fig 1 )," the eligibility of studies was assessed at three levels: general criteria, PICO criteria, and outcome measurements.For instance, we considered Population (i.e., cirrhosis and hepatic encephalopathy), Comparison (HE as the case and cirrhosis as the control), and Outcome (e.g., serum biomarkers).Given that our manuscript focused on evaluating observational studies, we intentionally refrained from providing a specific definition of intervention; instead, we exclusively included observational studies.
Regarding the comment, "I2 alone is not sufficient to assess heterogeneity between studies in statistical analysis.In cases of moderate or high heterogeneity, the methodological section of each study is usually re-reviewed to identify discrepancies, and sensitivity analyses excluding the discrepant study are typically performed (not done)," we precisely reviewed the methodology of each study and focused on the assessment methods, such as the evaluation of inflammatory markers using the Eliza method.
Additionally, we only included similar studies in the sub-analysis that differed in one variable.
I believe the major comments on the methodology were due to the concise nature of this part.The manuscript's word limitation was the main challenge we encountered.Following the aforementioned details, we have provided a comprehensive description of the comments in the supplementary document.